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Old 15-07-2019, 09:45   #1
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Alternator intermittently not charging

Hi all, I have had an issue recently with my alternator.

In the last week, it will be working one moment and then then it will stop charging the batteries. The first time it self corrected but now it’s just happened again.

I have checked the batteries and they are ok.

There is 16v coming from the +ve and -be terminals of the alternator.

Any hints where to check next? Would this suggest a faulty voltage regulator? Any way to confirm?
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Old 15-07-2019, 10:32   #2
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Re: Alternator intermittently not charging

It might be worth mentioning that I have a diode splitter. However the both the service and starter batteries are not being charged.
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Old 15-07-2019, 10:36   #3
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Re: Alternator intermittently not charging

I had a similar issue and it turned out a washer had fallen into the alt (!) and was grounding the diode pack
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Old 15-07-2019, 10:48   #4
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Re: Alternator intermittently not charging

Little bit of an update, I have 16v going from alternator to the split diode, 12.2v from the diode to service (same as measured at batt) and 12.6v at the diode to the starter (same as measured at batt).

Would that suggest it’s the diode?
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Old 15-07-2019, 10:54   #5
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Re: Alternator intermittently not charging

"Split diode" is not something Im familiar with. This is not part of the bridge rectifier, but external to the alternator?
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Old 15-07-2019, 10:56   #6
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Re: Alternator intermittently not charging

Quote:
Originally Posted by belizesailor View Post
"Split diode" is not something Im familiar with. This is not part of the bridge rectifier, but external to the alternator?


It’s for splitting the charge to the service and starter batteries as below.

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Old 15-07-2019, 11:22   #7
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Re: Alternator intermittently not charging

It does suggest there's a problem with the diode splitter, but 16v coming from the alternator also suggests a regulator problem. Depending on the type of diodes used in the splitter you should see a voltage drop of 0.2 VDC to 1.4 VDC on the output terminals.
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Old 15-07-2019, 11:43   #8
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Re: Alternator intermittently not charging

Quote:
Originally Posted by lawrence_craig View Post
It’s for splitting the charge to the service and starter batteries as below.

Attachment 196009
Interesting, Im more familiar with combiners/relays etc.

Doesnt sound like its part of the issue, so two other usual suspects are the bridge rectifier or the regulator.

A quick simple test is to VERY briefly touch a lead from the battery to the field terminal on the alternator with enine running. Alternator should go immediately to full output capacity at that RPM. If it goes full output then its likely the regulator. If not, then it could be the bridge rectifier. Test by testing each diode.

Another likely source is wiring...could be simple like a disconnected field/B+ wire. Check connections/continuity.
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Old 15-07-2019, 12:10   #9
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Re: Alternator intermittently not charging

It looks like bad diodes. Dropping 4 volts across the diodes is not normal. .4 volts maybe. Did you measure the voltage across the diodes? If that is okay then there is a bad connection/cable in the circuit. The reason the alternator voltage is so high is because, I assume, the sense voltage is read at the battery and is trying to get the voltage at the battery up. Better than diode splitters are the new charge combiner relays. Well, they aren't new but they did replace the diodes because of the voltage drop.
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Old 15-07-2019, 12:33   #10
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Re: Alternator intermittently not charging

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It looks like bad diodes. Dropping 4 volts across the diodes is not normal. .4 volts maybe. Did you measure the voltage across the diodes? If that is okay then there is a bad connection/cable in the circuit. The reason the alternator voltage is so high is because, I assume, the sense voltage is read at the battery and is trying to get the voltage at the battery up. Better than diode splitters are the new charge combiner relays. Well, they aren't new but they did replace the diodes because of the voltage drop.


I think you’re right. I have just literally bypassed the diode (alt straight connected to service batt) and I am now charging the service batteries. With 14.3v from alt and 13.8v to service.
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Old 15-07-2019, 16:26   #11
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Re: Alternator intermittently not charging

Good.

While you are replacing the diodes, hopefully with an ACR or equivalent, I would suggest that you take the time to clean and closely inspect all of your connections in the charging cct.

https://www.bluesea.com/products/cat...harging_Relays
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Old 15-07-2019, 19:48   #12
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Re: Alternator intermittently not charging

Sounds like the diode splitter if you have 16v in and 12.2 out. (Which is just the battery voltage.) Remove the alt wire from diode splitter and connect direct to one of the battery wires on the other posts . See if it will directly charge one battery.
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Old 15-07-2019, 19:50   #13
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Re: Alternator intermittently not charging

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Originally Posted by lawrence_craig View Post
I think you’re right. I have just literally bypassed the diode (alt straight connected to service batt) and I am now charging the service batteries. With 14.3v from alt and 13.8v to service.
If you are losing 0.5v from alt to battery. Probably time for a bigger alt cable too.
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Old 16-07-2019, 10:03   #14
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Re: Alternator intermittently not charging

Check the ground of the alternator. I had a pot connection at the block which raised the alternator negative by a couple volts above ground, limiting charging even though it had good output.
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Old 16-07-2019, 12:07   #15
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Re: Alternator intermittently not charging

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Originally Posted by lawrence_craig View Post
It’s for splitting the charge to the service and starter batteries as below.

Attachment 196009
That "splitter" is properly called a diode battery isolator. It has gone bad, causing high voltage drop. Voltage drop should be 0.7V with a silicon diode type and lower about 0.2V with MESFET isolator.

If the Vsense line to your alternator (usually a blue wire) is connected to the output of this faulty isolator or to the battery directly, it is telling the alternator to put out high voltage, 16V in your case, to try to raise the sense voltage to about 13.5V. This is only my suspicion based on your original and subsequent posts.

To test, you may simply bypass each diode temporarily moving the wire coming from alternator over to one of the battery wires connecting to the isolator. Start engine and remeasure voltages at that battery. Should now be around 13 5V depending on what you are using for voltage regulation on the alternator.
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